Nickel-chromium alloys containing rare earth metals and boron



2,838,394 Patented June 10,. 1958 NICKELCHRUMIUM ALLOYS CONTAINING RAREEARTH METALS AND BORON Bjorn Edwin and Sven A. G. Svensson,Hallstahammar,

Sweden, assignors to Alitiebolaget Kanthal, Hallstahammar, Sweden NoDrawing. Application January 23, 1956 Serial No. 560,905

Qlairns priority, application Sweden January 24, 1955 4 Claims. (Cl.75-171) Resistance materials of the kind specified may be divided intothree principal classes:

1) Alloys with a very low content of iron, iron being present merely asan impurity, and a chromium content of about 20 percent, the remainderbeing substantially all nickel.

(2) Alloys with an iron content of about 20 percent and a chromiumcontent of about 15 percent, the balance being substantially all nickel.

(3) Alloys with an iron content of about 45 percent and a chromiumcontent of about 20 percent, the remainder being substantially allnickel.

The invention to be described hereinafter is applicable in respect ofany one of these three classes of alloys.

A great number of patents of invention have already been granted onimprovements in resistance materials of the nickel-chromium type. Theobject of these prior inventions has been, by the addition of variouselements to improve the useful life of the alloy as measured accordingto the standard indicated by The American Society for Testing Materials(A. S. T. M.) in the publication Accelerated Life Test for MetallicMaterials, B. 7639. The prior disclosures are based in part on anaddition of zirconium and on combinations of zirconium and otherelements such as calcium, aluminium, etc., and in part on an addition ofrare-earth elements which may also be combined with other elements.Thus, the British patent specification No. 451,601 only discloses anaddition of rare-earth metals. The British patent specification No.488,926 discloses an addition of rare-earth metals in combination withcalcium, and the U. S. patent specification No. 2,687,956 disclosesadditions of rare-earth metals in combination with calcium andaluminium.

It has been found in attempts made by us to improve resistance alloys ofthe nickel-chromium type by the addition of rare-earth metals, that suchaddition involves the formation of a more dense and adhesive oxide. Thisimprovement of the oxide causes a substantially extended life asmeasured according to the A. S. T. M. testing standard above referredto. It has been stated, however, that, at elevated temperature, alloysof the nickel-chromium type containing additions of rare-earth elementsdisplay an increased trend towards oxidation along the grain boundariesthan do alloys without any addition of this kind. This phenomenon isparticularly pronounced in respect of the alloys of the class having alow iron content, but it also exists in alloys of higher percentages ofiron. It should be noted, however, that this grain boundary oxidationwill set in to any substantial extent only at temperatures which arehigher than those usually employed in life testing according to the A.S. T. M.

standard. Thus. the non-ferrous nickel-chromium alloys are tested at1175 C., whereas according to our testing practice, the marked trendtowards grain boundary oxidation has been observed only fromtemperatures in excess of 1200 C.

This grain boundary oxidation will decrease the utility of the materialas a resistance alloy, due in part to the fact that the oxidation willchange the electric resistance value of the alloy, and in part to thefact that the grain boundary oxidation causes the body of the materialto grow and thus to change its dimensions. Therefore, if it werepossible to reduce the tendency towards grain boundary oxidation, thiswould enable the quality of the material to be materially improved,particularly owing to the fact that the maximum operating temperaturecould be increased.

Our experiments have led to the unexpected result that an addition ofboron is very favorable and causes ,agreat reduction of the tendencytowards grain boundary oxidation. Since the boron added will in partvolatilize and in part react with other ingredients of the molten bath,the quantity of boron added will have to be substantially larger thanthat to be found afterwards in the alloy. It has been found that evenif, after boron having been added, only traces of this element are to befound in the alloy produced, nevertheless it has resulted in apronounced improvement of the resistance of the alloy to grain boundaryoxidation. The content of boron present in the alloy produced must notbe too high since this may reduce the useful life of the alloy.Therefore, it is considered as preferable with a final percentage ofboron not exceeding about 0.02. v

The quantity of rare-earth metals added to the alloy may be in the formof so-called misch metal containing approximately to cerium, 22 to 25%lanthanum, 15 to 17% neodymium and 8 to 10% of other rareearth metals.It is understood, however, that other alloys of rare-earth metals, orsuch metals uncombined, may also be used, as well as oxides of suchmetals in combination with a reducing agent adapted to transform theoxides into a free metallic state.

As a further alloying element we use silicon in percentages of 0.2 to2%.

It is practical in the production of resistance alloys of thenickel-chromium type to add metals adapted to cause deoxidation andoutgassing of the alloy, such as manganese, aluminium, zirconium,magnesium and calcium, as well as carbon stabilizing metals such asvanadium, titanium, columbium and tantalum. Further there will usuallybe a certain amount of cobalt present in the nickel as an impurity. Theexpression balance nickel as used hereinafter is to be interpreted asincluding the presence of the above-mentioned metals separately or inany combination and in percentages of the order of fractions of 1%.

The following examples of compositions alloyed in accordance with thepresent invention may be mentioned:

(1) 15 to 30% chromium, 0.2 to 2% silicon, 0.01 to 0.5% rare-earthmetals, traces to 0.02% boron, and the balance nickel.

(2) 10 to 25% chromium, 15 to 30% iron, 0.2 to 2% silicon, 0.01 to 0.5%rare-earth metals, traces to 0.02% boron, and the balance nickel.

(3) 10 to 25% chromium, 40 to iron, 0.2 to 2% silicon, 0.01 to 0.5%rare-earth metals, traces to 0.02% boron, and the balance nickel.

In order further to elucidate the signification of the addition ofboron, a brief report will be given of the results obtained from thetests carried out by us. The material to be tested was rolled intostrips 0.3 mm. in thickness and 10 mm. in width. From these stripssamples 35 mm. in length were cut and were heated in an electric furnaceto a temperature of 1210 C. The samples were operated cyclically for 2hours at this temperature and were allowed to cool for 1 /2 hours.However, every third cycle was chosen to last longer, the samples beingthen maintained at the high temperature for 15 /2 hours and were allowedto cool for 1 /2 hours. The total period of time during which thesamples were maintained at a high temperature was 425 hours. Used as ameasure of the grain boundary oxidation may be the elongation exhibitedby the samples after the above treatment.

The following results may be specified:

Alloy I.-19.5% chromium, 1.5% silicon, 0.07% cerium, balance nickel.Elongation after testing completed: 10.3%.

Alloy Il.l9.5% chromium, 1.5% silicon, 0.07% cerium, 0.002% boron,balance nickel. Elongation after testing completed: 2%.

In the production of both alloys as just described aluminium wasutilized as a deoxidizer.

What is claimed is:

1. A nickel-chromium alloy containing 10 to 30% chromium, to 60% iron,0.2 to 2% silicon and 0.01 to 0.5% rare-earth metals, and furthercharacterized in that it contains, in its finally produced condition,traces of boron amounting to a maximum content of 0.02%, and the balancenickel.

2. A nickel-chromium alloy containing to 30% chromium, 0.2 to 2% siliconand 0.01 to 0.5 rare-earth metals, and further characterized bycontaining, in its finally produced condition, traces of boron amountingto a maximum content of 0.02%, and the balance nickel.

3. A nickel-chromium alloy containing 10 to chromium, 15 to iron, 0.2 to2% silicon and 0.01 to 0.5 rare-earth metals, and further characterizedby containing, in its finally produced condition, traces of boronamounting to a maximum content of 0.02%, and the balance nickel.

4. A nickel-chromium alloy containing 10 to 25% chromium, to iron, 0.2to 2% silicon and 0.0l to 0.5 rare-earth metals, and furthercharacterized by containing, in its finally produced condition, tracesof boron amounting to a maximum content of 0.02%, and the balancenickel.

References Cited in the file of this patent UNITED STATES PATENTS2,104,836 Hessenbruch Jan. 11, 1938 2,255,895 Pfeil Sept. 16, 19412,289,640 Fetz July 14, 1942

1. A NICKEL-CHROMIUM ALLOY CONTAINING 10 TO 30% CHROMIUM, 0 TO 60% IRON, 0.2 TO 2% SILICON AND 0.01 TO 0.5% RARE-EARTH METALS, AND FURTHER CHARACTERIZED IN BORON AMOUNTING TO A MAXIMUM CONTENT OF 0.02%, AND THE BALANCE NICKEL. 